1. Field of the Invention
The present invention relates to an optical module for transmitting or receiving plural optical signals, and particularly, to an optical module capable of monitoring emitted light of light-emitting elements.
2. Description of the Related Art
In an optical communication system, there is an optical module that serves to monitor LD-emitted light.
FIG. 16 shows a conventional optical module 90 that comprises a laser diode (LD) 91, a mirror 92 provided at a specified angle to the light-emitting plane of the LD for allowing an optical signal to be transmitted through but partially reflected off the mirror, and a monitoring light-receiving element array 93 for receiving an optical signal reflected off the mirror 92 (See JP-A-2002-107582, for example). The monitoring light-receiving element array 93 is provided on a substrate 94 on which is packaged the LD 91. The mirror 92 is formed of a glass plate that reflects a few percents of LD-emitted light, and is arranged so that an incident angle of the optical signal on the glass plate is below a critical angle to allow most of the LD-emitted light to be transmitted therethrough.
Most (L′ in the figure) of an optical signal L for communication emitted from the LD 91, is transmitted through the glass plate, and a few percents thereof (L″ in the figure) are reflected off it. The reflected light L″ is received by the monitoring light-receiving element array 93 provided in the reflection direction.
Since a few percents of the optical signal emitted from the LD 91 are reflected and detected, it is possible to monitor whether the LD 91 is operating normally.
Because the optical module of FIG. 16 is a one-channel optical module with one monitoring light-receiving element that receives an optical signal emitted from one LD and its monitored light, it cannot be applied to an optical module with an LD array comprising plural LDs arrayed in parallel to each other to emit plural optical signals.
Because it is generally difficult to emit light rearwards from a surface emitting LD, light emitted forwards has to be separated with some method into an optical signal for communication and an optical signal to be monitored. A method, which causes reflection by providing a glass plate in front of the surface emitting LD, causes expansion of the spot of the light returned by being reflected off the glass plate. Particularly, in a parallel transceiver with plural LDs arranged at a narrow pitch, there is the problem that optical signals to be monitored interfere with each other between mutually adjacent channels.
This therefore makes it impossible to discretely monitor respective optical signals emitted from the LDs of the LD array. In the optical module that transmits/receives optical signals of the multichannel, it is impossible to monitor the optical signal emitted from each light-emitting element, to determine which channel is malfunctioning.